In a typical hydraulic power operated system, a hydraulic pump, which may produce either a fixed flow rate or a variable flow rate, provides a source of pressurized hydraulic fluid to power multiple fluid operated devices which are plumbed into the system. In a lifting application, the fluid power devices may be two or more hydraulic cylinders. In another application, for example dispensing salt from the dump box of a snow removal truck, the fluid power devices may be two hydraulic motors, one for operating an auger to move the salt from the box to a rotary spreader and another for rotating the spreader so as to distribute the salt from the auger discharge across a roadway. Different types of fluid power devices may be combined depending upon the application and the effect desired, such applications being virtually limitless.
In applications where two or more fluid power devices are employed, the fluid power devices are almost always controlled, either manual remote or automatically, so that they work in synchronization with one another. For example, in the lifting application described above, when lifting a load the cylinders must advance at the same rate in order to keep the load level, or at least must advance at a fixed rate in proportion to one another so as to keep the orientation of the load from shifting. Similarly, in the salt dispensing application example described above, the speed that the auger delivers salt to the rotary table must be controlled in relation to the speed of rotation of the table.
A problem in controlling the relative speeds of two or more fluid power devices occurs when the flow demand of the devices exceeds the maximum flow capacity of the pump supplying the system. Under these circumstances, the fluid power devices having the lower pressures are favored over the higher pressure devices. Therefore, the lower pressure devices receive more flow and the higher pressure devices receive less. This results in the relative speeds of the devices changing, with the higher pressure devices slowing down in relation to the lower pressure devices, which is undesirable.